Cellulosic biomass is a significant renewable resource for the generation of fermentable sugars. These sugars can be used as reactants in various metabolic processes, including fermentation, to produce biofuels, chemical compounds, and other commercially valuable end-products. While the fermentation of simple sugars such as glucose to ethanol is relatively straightforward, the efficient conversion of cellulosic biomass to fermentable sugars is challenging. See, e.g., Ladisch et al., 1983, Enzyme Microb. Technol. 5:82. Cellulose may be pretreated chemically, mechanically, enzymatically or in other ways to increase the susceptibility of cellulose to hydrolysis. Such pretreatment may be followed by the enzymatic conversion of cellulose to cellobiose, cello-oligosaccharides, glucose, and other sugars and sugar polymers, using enzymes that break down the β-1-4 glycosidic bonds of cellulose. These enzymes are collectively referred to as “cellulases.”
Cellulases are divided into three sub-categories of enzymes: 1,4-β-D-glucan glucanohydrolase (“endoglucanase” or “EG”); 1,4-β-D-glucan cellobiohydrolase (“exoglucanase”, “cellobiohydrolase”, or “CBH”); and β-D-glucoside-glucohydrolase (“β-glucosidase”, “cellobiase” or “BGL”). Endoglucanases break internal bonds and disrupt the crystalline structure of cellulose, exposing individual cellulose polysaccharide chains (“glucans”). Cellobiohydrolases incrementally shorten the glucan molecules, releasing mainly cellobiose units (a water-soluble β-1,4-linked dimer of glucose) as well as glucose, cellotriose, and cellotetrose. There are two main types of cellobiohydrolases: Type 1 cellobiohydrolases are members of the Glycoside Hydrolase Family 7, and cleave processively from the reducing end of cellulose chains. Type 2 cellobiohydrolases cleave processively from the non-reducing end of cellulose. β-glucosidases split cellobiose into glucose monomers.
Cellulases with improved properties for use in processing cellulosic biomass would reduce costs and increase the efficiency of production of biofuels and other commercially valuable compounds.